The present invention generally relates to pseudo vertical synchronizing signal producing circuits, and more particularly to a pseudo vertical synchronizing signal producing circuit for producing a pseudo vertical synchronizing signal which is used in place of a regular vertical synchronizing signal in a special reproduction mode of a magnetic recording and reproducing apparatus.
There are various helical scan type magnetic recording and reproducing apparatuses (hereinafter referred to as video tape recorders or simply VTRs) for recording and reproducing a video signal on and from a magnetic tape by use of rotary video heads. As systems in which the rotary video heads to be used are changed depending on a moving speed of the tape, there are a 4-head double-gap system, a 4-head 90.degree. system, a 3-head double-gap system and the like.
For example, a VTR employing the 4-head double-gap system comprises a rotary drum mounted with a pair of mutually confronting standard play heads and a pair of mutually confronting expanded play heads. Each standard play head lags a corresponding expanded play head in a rotating direction of the rotary drum by a distance corresponding to two horizontal scanning periods on an outer peripheral surface of the rotary drum, for example. The standard play heads are used in a standard play mode, and the expanded play heads are used in an expanded play mode in which the tape is moved at a speed slower than that at the time of the standard play mode.
Two heads having gaps of the same azimuth angle, that is, a first standard play head and a first expanded play head leading a second standard play head which confronts the first standard play head, are used when carryinig out a still reproduction in the standard play mode. Remaining two heads having gaps of the same azimuth angle, that is, the second standard play head and a second expanded play head leading the first standard play head which confronts the second standard play head, are used when carrying out a still reproduction in the expanded play mode. As a result, a level of a frequency modulated signal reproduced from the tape during the still reproduction if high, and it is possible to obtain a satisfactory still picture in the standard and expanded play modes.
The level of the frequency modulated signal reproduced in a normal reproduction mode is approximately constant. However, in a special reproduction mode, the heads scan obliquely to the tracks on the tape and traverse the tracks. As a result, the level of the frequency modulated signal reproduced in the special reproduction mode changes. When such a reproduced frequency modulated signal obtained in the special reproduction mode is detected, a vertical synchronizing signal may not be reproduced due to noise, and in this case, the vertical synchronism is disturbed in a reproduced picture.
In order to prevent the vertical synchronism in the reproduced picture from being disturbed by the absence of the reproduced vertical synchronizing signal, a pseudo vertical synchronizing signal is produced in a pseudo vertical synchronizing signal producing circuit. The pseudo vertical synchronizing signal is produced from a drum switching pulse signal which is used to switch outputs of the heads in the special reproduction mode, and the pseudo vertical synchronizing signal is inserted into the video signal in a vicinity of the regular vertical synchronizing signal. However, as will be described later on in the specification in conjunction with drawings, a conventional pseudo vertical synchronizing signal producing circuit uses monostable multivibrators respectively having an external variable resistor coupled externally thereto for each mode of the VTR. As a result, there are problems in that the number of external variable resistors is large, and that it is difficult to manufacture the pseudo vertical synchronizing signal producing circuuit in the form of an integrated circuit.